Threading control method and apparatus therefor

ABSTRACT

A threading control method and apparatus therefor is provided, in which, when carrying out thread-cut machining operations several times at one position, synchronized with main-spindle rotation and controlling movement of a threading spindle, deviations in thread grooves are prevented without correcting programmed commands, machining accuracy is improved, and tool life extended. 
     When a main-spindle single-rotation reference signal and a main-spindle position counter are input to the numerical control apparatus, the present main-spindle position is computed by a main-spindle position computing means  103 , the difference between the main-spindle single-rotation reference signal and control cycles is obtained, as a correction amount  108 , from the computed main-spindle position, by a main-spindle position-correcting means  104  and a correction is done. A threading-spindle interpolation starting detection means  105  monitors whether the main-spindle single-rotation reference signal and the control cycles are synchronous, and when synchronous, interpolation for the threading spindle is started by interpolation means  106  for each spindle.

TECHNICAL FIELD

The present invention relates to a threading control method and anapparatus therefor.

BACKGROUND ART

In conventional numerical control apparatuses (in what follows,numerical control is referred to as NC), a main spindle holding aworkpiece is given a constant rotation, the amount of rotation of themain spindle is detected by an encoder attached to the main spindle,threading-spindle movement amount proportional to a prescribed threadlead is computed, and movement control is carried out.

Further, when a thread is made in the workpiece, in general, thread-cutmachining is repeated several times while varying tool cutting amount,and this controls starting of interpolation for the threading spindle,with a main-spindle single-rotation reference-position signal asreference. However, since the main-spindle single-rotation referenceposition signal and NC control cycle IT are not synchronous, in caseswhere two or more machining repetitions are carried out for the samethread, there have been occurrences of variations in the actual cuttingposition.

Further, since there are variations in the cut-finish path near the endof the threading, due to variations produced at start of threading, anincomplete thread portion at the thread ending becomes long, and it isnecessary to take into account the incomplete thread length includingthe variation portion. Further, due to fluctuations in cutting load dueto variations in the cut-finish path close to the end point of thethreading, thread accuracy near the end point degrades, and due to thefluctuations in this cutting load, there have been cases where tool lifehas been shortened.

Using FIG. 6 to FIG. 8, a detailed description of these issues is asfollows.

As illustrated in FIG. 8, thread-cut machining is implemented by givingthe workpiece, attached to the main spindle, a constant rotation, and bymoving a cutting tool from a threading starting position, set at a fixedprescribed position, in the direction of the threading spindle.

In conventional threading control, as illustrated in FIG. 6, themain-spindle position, when threading, is obtained by a main-spindleposition counter in which, when threading, a counting operation isstarted with the main-spindle single-rotation reference position signal,which is output by the encoder installed on the main spindle every timethe main spindle makes one rotation, as a trigger. Further, whenthreading, regarding this main-spindle position counter, output isgenerated by the main-spindle position counter that is used for controlof each rotation, the count is cleared at a prescribed count number, andthe count restarted. The threading control controls the starting ofthreading-spindle interpolation based on the main-spindlesingle-rotation reference position signal; at this time, the amount thethreading spindle moves is computed in the interpolation processing ofthe NC apparatus, executed at a constant cyclic interval (for example,10 msec), and the amount the threading spindle moves at the firstinterpolation is:FΔTo=(ΔPo÷P)×L

-   (ΔPo: first-time main-spindle position variation amount [number of    pulses],-   P: number of pulses per main-spindle single rotation,-   L: thread lead    Here, since the main-spindle rotation cycle and the control cycle IT    are asynchronous, variations occur in the value of ΔPo and also in    the threading-spindle movement amount, FΔTo, at the first    interpolation. These variations become the variations at the start    of threading, and finally, as illustrated in FIG. 8, become the    variations at the start of cut-finishing, so that the cut-finish    path is no longer constant. FIG. 7 illustrates how the    threading-spindle movement amount, FΔT₁, at the final interpolation    varies when the threading-spindle movement amount, FΔTo, at the    first interpolation varies.

Technology for solving these types of problems is disclosed in JapaneseLaid-Open Patent Publication 1993-46236, consisting of altering thecontrol cycle for a servo, so as to have synchronicity with themain-spindle single-rotation reference signal However, since the abovetechnology requires special hardware (H/W) apparatus in order to changethe control cycle just before the thread-cut machining, there aredisadvantages in that the H/W configuration becomes complicated.

DISCLOSURE OF INVENTION

In the light of these types of issues, the present invention has as anobject the provision of a threading control method and apparatustherefor, that can perform thread-cut machining with good accuracy,without the occurrence of variations at the start of threading, even incases where thread-cut machining is repeated several times whilealtering tool cutting amount, without changing the conventional H/W.

In order to solve the conventional problems described above, thethreading control method related to the present invention comprises astep of computing the present position of a main spindle, a step ofgenerating a main-spindle position correction amount in order to make asingle-rotation reference signal of the main spindle synchronous with acontrol cycle, based on the single-rotation reference signal of the mainspindle and the computed present position of the main spindle, and ofcorrecting, by this main-spindle position correction amount, theposition of the main spindle so that the single-rotation referencesignal of the main spindle and the control cycle are synchronized, astep of confirming the synchronization of the control cycle and themain-spindle single-rotation reference signal whose position has beencorrected, and a step of outputting a command to the threading spindlewhen the main-spindle single-rotation reference signal and the controlcycle are synchronized.

As a result it is possible, without changing the conventional H/W, tomake the control cycles conform with the rotation cycle of the mainspindle by correcting the position of the main spindle, from the presentmain-spindle position counter value and the main-spindle single-rotationreference position. In this way, even in cases where the thread-cutmachining is repeated several times while changing the tool cuttingamount, since variations are not produced when starting threading andclose to the end point of the threading, the thread-cut machiningaccuracy is improved. Since the threading end point path is constant,changes in cutting load are reduced and the tool life is extended.

Furthermore, the threading control method of this invention is such thatthe position of the main spindle is corrected in a direction in whichthe rotational frequency of the main spindle diminishes.

Thus, the correction can be done below the maximum rotational frequencyof the main spindle, so that the thread-cut machining can be carried outsafely.

Further, the threading control method of this invention is such that,when the deviation between the single-rotation reference signal of themain spindle and the control cycle is below a prescribed value, and therotational frequency of the main spindle is below a designated value,the position of the main spindle is corrected in the direction in whichthe rotational frequency of the main spindle increases.

As a result, the time necessary for correction is reduced and the cycletime is shortened.

Furthermore, the threading control method of this invention is such thatthe main-spindle position correction amount is computed to be below themaximum correction amount of the main spindle, so that the variations inrotation of the main spindle are within predetermined variations.

As a result, main-spindle speed variations can be restrained and theoccurrence of inaccuracies and main-spindle alarms can be restrained.

Additionally, the threading control method of this invention is suchthat the main-spindle position correction amount includes the threadingstarting angle.

As a result, it is possible to start thread-cut machining from anarbitrary angle.

A threading control apparatus related to the present invention comprisesa main-spindle position computing means for computing the presentposition of the main spindle, a main-spindle position correcting meansfor generating a main-spindle position correction amount in order tomake a single-rotation reference signal of the main spindle synchronouswith a control cycle, based on the single-rotation reference signal ofthe main spindle and the main-spindle present position computed by themain-spindle position computation means, and for correcting, by thismain-spindle position correction amount, the position of the mainspindle so that the single-rotation reference signal of the main spindleand the control cycle are synchronized, a threading-spindleinterpolation start detection means for confirming the synchronizationof the control cycle and the main-spindle single-rotation referencesignal whose position has been corrected, and an interpolation means foreach spindle for outputting a command to the threading spindle when themain-spindle single-rotation reference signal and the control cycle aresynchronized.

As a result it is possible, without changing the conventional H/W, tomake the control cycles conform with the rotational cycle of the mainspindle, by correcting the position of the main spindle from the presentmain-spindle position counter value and the main-spindle single-rotationreference position.

In this way, even in cases where the thread-cut machining is repeatedseveral times while changing the tool cutting amount, since variationsare not produced when starting threading and close to the end point ofthe threading, the thread-cut machining accuracy is improved. Since thethreading end point path is constant, changes in cutting load arereduced and the tool life is extended.

Furthermore, the threading control apparatus of this invention is suchthat the main-spindle position correcting means corrects the position ofthe main spindle in a direction in which the rotational frequency of themain spindle diminishes.

Thus, correction can be done below the maximum rotational frequency ofthe main spindle, so that thread-cut machining can be carried oursafely.

Further, the threading control apparatus of this invention is such that,when the deviation between the single-rotation reference signal of themain spindle and the control cycle is below a prescribed value, and therotational frequency of the main spindle is below a designated value,the main-spindle position correcting means corrects the position of themain spindle in the direction in which the rotational frequency of themain spindle increases.

As a result, the time necessary for correction is reduced and the cycletime is shortened.

Furthermore, the threading control apparatus of this invention is suchthat the main-spindle position correcting means performs computationwith the correction amount for the main-spindle position below themaximum correction amount, so that the main-spindle rotationalvariations are within predetermined variations.

As a result, the main-spindle speed variations can be restrained and theoccurrence of inaccuracies and main-spindle alarms can be restrained.

Additionally, the numerical control apparatus of this invention is suchthat the main-spindle position correction amount computed by themain-spindle position correcting means includes the threading startingangle.

As a result, it is possible to start thread-cut machining from anarbitrary angle.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating one configuration example of anNC apparatus related to Embodiment 1 of the present invention;

FIG. 2 is an explanatory diagram for threading control related toEmbodiment 1 of the present invention;

FIG. 3 is a diagram explaining an effect related to Embodiment 1 of

FIG. 4 is an explanatory diagram for processing of the threading controlrelated to Embodiment 1 of the present invention;

FIG. 5 is a flowchart illustrating processing steps for the threadingcontrol related to Embodiment 1 of the present invention;

FIG. 6 is an explanatory diagram for conventional threading control;

FIG. 7 is an explanatory diagram of examples of threading positionvariations with the conventional threading control; and

FIG. 8 is an explanatory diagram of a threading method.

BEST MODE FOR CARRYING OUT THE INVENTION Embodiment 1

Embodiment 1 of the present invention is described below, referring toFIGS. 1 through 5.

In Embodiment 1 related to the present invention, in order to makemain-spindle rotation cycle be in conformance with control cycle IT, asillustrated in FIG. 2, main-spindle rotation speed is slowed down for amoment so as to correct the deviance between the control cycle IT and amain-spindle single-rotation reference position signal, and themain-spindle rotation cycle is made synchronous with the control cycleIT. The arrangement is such that, after it has been confirmed that themain-spindle single-rotation reference position signal and the controlcycle IT are in conformance, movement amount for the threading spindleis generated. In this way, as illustrated in FIG. 3, even if thethread-cut machining is repeated several times, the spindle movementamount at threading start and at threading completion is alwaysconstant, and variations in movement amount in the first interpolationat threading start are eliminated.

Details for performing the control described above are now explained.

FIG. 1 is a block diagram illustrating an overall configuration for anNC apparatus related to Embodiment 1 of the present invention.

In FIG. 1, one block at a time is read in by a program analysis means101 from a machining program 110 housed in a memory, and movementamount, speed and the like are read out together with a G command. Basedon information analyzed in this program analysis means 101, in aninterpolation processing unit 102 where processing takes place at thefixed control cycle IT (for example, 10 msec), the movement amount isoutput per unit of time (for example 10 msec) for each spindle (the mainspindle for rotating the workpiece and a spindle for moving thethreading tool.)

The main-spindle single-rotation reference signal and the main-spindleposition counter when threading (the arrangement being made by themain-spindle position counter used for forwarding-control at eachrotation, the main spindle being given a single rotation once, and acounting operation being started with the main-spindle single-rotationreference position signal outputted by an encoder installed on the mainspindle, as a trigger, the count being cleared every predeterminednumber of counts and the count being restarted again) are input in theinterpolation processing unit 102 where processing takes place at afixed cycle interval, the present position (angle) of the main spindleis computed by a main-spindle position computing means 103, from thiscomputed main-spindle position, the deviation between the main-spindlesingle-rotation reference signal and the control cycle is obtained, bythe main-spindle position-correcting means 104, as a correction amount108, and this correction amount 108 is split up so as to be accommodatedwithin the main-spindle maximum correction amount 112, and is outputted.

The main-spindle maximum correction amount 112 is set, considering thatif the correction amount is output to the main-spindle once, therotational speed variation of the main spindle becomes large and analarm occurs, and, with the maximum possible correction value percontrol cycle unit, is equivalent to a main-spindle movement amount FΔTs(see FIG. 2) per control cycle unit.

A threading-spindle interpolation starting detection means 105, monitorswhether the main-spindle single-rotation reference signal and thecontrol cycle are synchronous, and in cases of synchronicity,interpolation for the threading spindle is carried out by aninterpolation means 106 for each spindle.

By adding a threading starting angle 114 to the correction amount 108obtained by the main-spindle position-correcting means 104, it ispossible to shift the threading starting position.

However, the shifting of the threading starting position is for casessuch as when a reformed thread is being made (a thread in whichpreviously cut thread is re-threaded for use); in cases where newthreading is carried out in a workpiece that has not been threaded,shifting of this threading starting position is normally not done.

Next, in Embodiment 1, after the main-spindle rotation cycle and thecontrol cycles are synchronized and it is confirmed that the referenceposition signal and the control cycles are in conformance, theprocessing steps in generating the movement amount for the threadingspindle are explained, referring mainly to the flowchart illustrated inFIG. 5.

In the interpolation processing unit 102 where processing is repeatedlycarried out every fixed cycle (for example, every 10 msec), based oninformation from each single block in the machining program 110, outputby the program analysis means 101, a judgment is made as to whether thepresent command is the threading command (step S100), and in cases whereit is not the threading command, processing is finished at that point.

In cases where the present command is the threading command, referenceis made to a synchronization completion flag 107 representing whetherthe main-spindle rotation cycle and the control cycle, illustrated inFIG. 1, are in synchronization (step S110). Here, in cases where thesynchronization completion flag 107 is on, that is, the main-spindlerotation cycle and the control cycle are in synchronization, theinterpolation for the threading spindle is started and thethreading-spindle movement amount is obtained (Step S190).FΔTz=(ΔPz÷P)×L

-   (ΔPz: first-time main-spindle position variation amount [number of    pulses], P: number of pulses for a single rotation of the main    spindle, L: thread lead-   The obtained movement amount FΔTz of the threading spindle is output    as the command value (Step S200).

In cases where the synchronization completion flag 107 is off, that is,the main-spindle rotation cycle and the control cycle are asynchronous,in order to obtain the point A, being the present main-spindle position,illustrated in FIG. 4, (the interpolation starting signal for the mainspindle is monitored for the position where it coincides with thismain-spindle interpolation starting signal), as the clamping valuewithin one main-spindle rotation, and make a correction, computation iscarried out as: the present main-spindle position counter value (pointA) minus the previous reference position (point B: grid length). (StepS120).

The value obtained from this computed result is the deviation amountbetween the main-spindle single-rotation reference position and thecontrol cycle IT (=the correction amount 108).

The threading starting angle 114 illustrated in FIG. 1 is added to thepreviously obtained value of the point A clamped within a main-spindlesingle-rotation (Step S130), and a comparison is made (Step S140) withthe main-spindle single-rotation reference position, point C (as will bedescribed below, this should be a position in conformance with thecontrol cycle IT, after a controlled slowing of the speed of themain-spindle rotation in accordance with the correction amount 108.)Here, in cases where the compared result is one of conformance, thesynchronization completion flag 107 is made ON (Step S180).

In cases where the threading starting angle 114 command is issued as anangle, this angle is added after it is converted to a counter value.Further, in cases where it is not necessary to shift the threadingstarting position, Step S130 is not necessary.

In Step S140, since before starting the threading-spindle interpolation,the main-spindle single-rotation reference signal and the control cycleare generally not in synchronization, the procedure normally moves toStep S150.

Next, in cases where the comparison result is different, the previouslyobtained present main-spindle position counter value, clamped within themain-spindle single rotation (=the correction amount 108) (point A) iscompared with the main-spindle maximum correction amount 112 (StepS150); in cases where this present main-spindle position counter valueis less than the main-spindle maximum correction amount 112, themain-spindle position counter value is subtracted from the main-spindlemovement amount FΔTs per control cycle unit, obtained from themain-spindle command rotation number, as illustrated in FIG. 2, and thissubtraction result is made the main-spindle movement amount FΔTsc percontrol cycle unit (Step S170), and the speed of the main spindle isdecreased. In cases where the present main-spindle position countervalue, clamped within the main-spindle single rotation is larger thanthe main-spindle maximum correction amount 112 of FIG. 1, themain-spindle maximum correction amount 112 is subtracted from themain-spindle movement amount FΔTs per control cycle unit, and thissubtraction result is made the main-spindle movement amount FΔTsc percontrol cycle unit (Step S160), and the speed of the main spindle isdecreased. Regarding the portion of the correction not effected in StepS160, processing of Steps S100–S160 is repeated, and finally processingmoves to Step S170, so that the rotation cycle of the main spindle andthe control cycle are in conformance.

In this way, in Embodiment 1, based on the present main-spindle positioncounter value and the main-spindle single-rotation reference position,the speed of the main spindle is decreased and the position of the mainspindle is corrected, so that it is possible to make the control cyclebe in conformance with the main-spindle rotation cycle, and in this way,the threading spindle movement amount, when threading is started, can bemade to be always constant.

Embodiment 2

In Embodiment 1 described above, the position of the main spindle wascorrected in the direction in which the main-spindle rotation decreases;however, in cases where the present main-spindle position counter value(point A) is closer to the counter-rotation direction than the referenceposition, and the difference between the present main-spindle positioncounter value (point A) and the reference position, that is, thecorrection value, R, which is equal to the pulse value for onemain-spindle rotation minus the position counter value of a singlerotation, is within an increase-range upper-limiting value 113 for themain-spindle rotation, illustrated in FIG. 1 (a value at which, evenwith the correction value R added to the main-spindle movement amountFΔTs, speed variation is not large and the main-spindle alarm does notoccur), and even with the correction amount R added to the main-spindlemovement amount FΔTs, a given main-spindle rotational frequency is notexceeded, and taking the main-spindle movement amount FΔTs per controlcycle unit, to which the correction value R is added, as themain-spindle movement amount FΔTsc per control cycle unit, by increasingthe main-spindle speed, the main-spindle rotation cycle may be made tobe in conformance with the control cycle.

INDUSTRIAL APPLICABILITY

The threading control method and apparatus therefor, related to thepresent invention as described above is suitable for use as a threadingcontrol method and apparatus therefor in cases where machining isrepeated two or more times for the same thread.

1. A threading control method for moving a cutting tool and a workpiecein synchronicity with rotation of a main spindle to machine threadgrooves in the workpiece, the threading control method comprising:computing the present position of the main spindle; generating acorrection amount of the main-spindle position in order to make asingle-rotation reference signal of the main spindle synchronous with acontrol cycle, based on the single-rotation reference signal of the mainspindle and the computed present position of the main spindle, andcorrecting, by this correction amount of the main-spindle position, theposition of the main spindle so that the single-rotation referencesignal of the main spindle and the control cycle are synchronized;confirming the synchronization of the control cycle and the cycle of thesingle-rotation reference signal of the main spindle whose position hasbeen corrected; and outputting a command to a threading spindle when themain-spindle single-rotation reference signal and the control cycle aresynchronized.
 2. The threading control method according to claim 1,wherein the main-spindle position is corrected in a direction in whichthe main-spindle rotational frequency decreases.
 3. The threadingcontrol method according to claim 1, wherein when the deviation betweenthe main-spindle single-rotation reference signal and the control cycleare below a prescribed value, and the rotational frequency of the mainspindle is below a designated value, then the position of the mainspindle is corrected in a direction in which the rotational frequency ofthe main spindle increases.
 4. The threading control method according toclaim 2, wherein the correction amount of the main-spindle position iscomputed to be below a main-spindle maximum correction, in order thatvariations in the main-spindle rotation be within a prescribed variationrange.
 5. The threading control method according to claim 1, wherein thecorrection amount of the main-spindle position includes a threadingstart angle.
 6. A threading control apparatus for moving a cutting tooland a workpiece in synchronicity with rotation of a main spindle tomachine thread grooves in the workpiece, the threading control apparatuscomprising a main-spindle position computing means configured to computethe present position of the main spindle; a main-spindle positioncorrecting means configured to generate a correction amount of themain-spindle position in order to make a single-rotation referencesignal of the main spindle synchronous with a control cycle, based onthe single-rotation reference signal of the main spindle and the presentmain-spindle position computed by the main-spindle position computingmeans, and configured to correct, by this correction amount of themain-spindle position, the position of the main spindle so that thesingle-rotation reference signal of the main spindle and the controlcycle are synchronized; a threading-spindle interpolation startingdetection means configured to confirm the synchronization of the controlcycle and the cycle of the single-rotation reference signal of the mainspindle whose position has been corrected; and an interpolation meansfor each spindle configured to output a command to the threading spindlewhen the main-spindle single-rotation reference signal and the controlcycle are synchronized.
 7. The threading control apparatus according toclaim 6, wherein the main-spindle position correcting means corrects themain-spindle position in a direction in which the rotational frequencyof the main spindle decreases.
 8. The threading control apparatusaccording to claim 6, wherein when the deviation between themain-spindle single-rotation reference signal and the control cycle andthe main spindle is below a prescribed value, and the rotationalfrequency of the main spindle is below a designated value, then themain-spindle position correcting means corrects the main spindleposition in a direction in which the rotational frequency of the mainspindle increases.
 9. The threading control apparatus according to claim6, wherein the main-spindle position correcting means computes thecorrection amount of the main-spindle position to be below a maximumcorrection, in order that variations in the main-spindle rotation bewithin a prescribed variation range.
 10. The threading control apparatusaccording to claim 6, wherein the correction amount of the main-spindleposition computed by the main-spindle position correcting means includesa threading start angle.